So that we don't have to learn yet another version control system. We've already learned git and github, why switch now? Mozilla's Mercurial repo is mirrored on github, and our repo is a fork of this. The main github repo (e.g., https://github.com/doublec/mozilla-central) gets updated regularly, so you can pull from it to keep your fork in sync.

See https://developer.mozilla.org/En/Developer_Guide/Build_Instructions